In a typical transportation system operative with comma-free, binary coded vehicle control signals, vehicle speed command information is transmitted on multiple message frequencies (e.g. 5 KHz and 10 KHz) which represents binary message units "1" and "0". A digital frequency modulation method (FM), frequency shift key modulation method (FSK), or phase shift key modulation method (PSK) is employed to carry binary coded, speed command information to the transit vehicles. Before the information transmitted on the message frequencies can be utilized, it must be decoded according to the particular modulation method used by the system. This decoding must be performed in a manner such that extraneous signals which are received will not cause the vehicle to operate in an unsafe manner.
In attempting to achieve this safety standard, vehicle command signal decoders of the prior art used a limiting amplifier to establish "capture effect" by which a coded message frequency of large amplitude could screen out extraneous signals of lower amplitude. The gain of the limiting amplifier was high enough that any input signal of a predetermined minimum amplitude would result in an output of limited maximum amplitude. Therefore, as long as the amplitude of the message frequency was large enough to maintain the predetermined minimum amplitude of the input signal while, at the same time, offsetting any other input signals, the limited amplitude output of the limiting amplifier was determined by the message frequency. For example, an input message frequency whose amplitude was larger than twice as large as the amplitude of any other input signal (noise or transient filter responses) would screen out the other signals so that the limited maximum output of the limiting amplifier would be determined by the input message frequency.
The problem with the prior art decoders was caused by the high gain of the limiting amplifier. For example, if the output of a filter in the prior art decoder were to contain a transient response while the message frequency being transmitted was outside the bandwidth of the filter, the limiting amplifier could recognize this transient response as a valid input and amplify it so that the detector would detect an inaccurate message unit causing the decoder to decode inaccurate information. The high gain of the limiting amplifier could also cause errors in detection where the large amplitude message frequency was lost. When the large amplitude message frequency was not present to screen out the smaller, extraneous signals, the limiting amplifier could amplify these extraneous signals also causing the decoder to decode inaccurate information. Also, the failure of a filter could allow a signal outside the bandwidth of the filter to reach the input of the limiting amplifier where it could be amplified and again cause the decoder to decode inaccurate information.
To the extent that the decoding errors caused by the use of a limiting amplifier in the prior art decoder manifested themselves, transportation vehicle control systems that used this prior art design had limited safety margin. It was the purpose of the present invention to provide a transportation vehicle signal decoder that could significantly improve transportation system safety by eliminating the decoding errors caused by limiting amplifiers.